The present invention generally relates to intra-cavity spectroscopy. More specifically, the invention is directed to methods and apparatus for identifying a compound which is exposed to a light wave resonating within a laser cavity.
Absorption cell spectroscopy has been used for many years. According to some conventional methods, white light is coupled through an absorption cell and subsequently scanned by a monochrometer. When absorption occurs in the cell at a particular wavelength, as measured by the output of the monochrometer, there is a decrease in the intensity of the light in accordance with Beer's law given by: EQU I(L,.lambda.)=I.sub.0 (0,.lambda.)e.sup.-.alpha..sbsb..lambda..sup.L
where L is the length of the absorption cell. The intensity of the light entering the absorption cell as a function of wavelength is I.sub.0 (0,.lambda.) and the light exiting the absorption cell is I(L,.lambda.). The absorption coefficient .alpha..sub..lambda. depends upon the wavelength, the number of absorbers and the molecular cross section of the absorption species.
According to other conventional methods, instead of a white light source, a tunable laser source is employed to couple laser light through the absorption chamber. When the laser wavelength coincides with an absorption wavelength of the species in the absorption chamber, the intensity of the light passing through the absorption chamber decreases in accord with Beer's law.
Since the sensitivity of the detection of the species present in the absorption cell is exponentially dependent upon the length of the absorption cell, this sensitivity can be increased by increasing the path length of light though the absorption cell. To this end, some prior approaches attempt to increase the sensitivity by passing the light through the absorption cell a plurality of times.
According to other conventional methods, the absorption cell is located inside the laser cavity. If a species which absorbs light at the lasing frequency is placed in the laser cavity, the output intensity of the laser drops. Photons make many round trips before exiting the cavity. The effective absorption interaction path length is significantly increased by this technique. This approach is generally known to increase the sensitivity of measurements by several orders of magnitude.
Several prior art approaches have been employed to create laser cavities which incorporate absorption cells. By way of example, fiber optic lasers have recently been employed. According to one conventional method, a neodymium doped fiber of approximately two meters in length is pumped by an argon ion laser or by a diode laser. An open portion of the laser cavity, which is approximately 35 cm long, is defined between two dielectric mirrors and is filled with a sample having an unknown quantity of an absorbing species (water vapor). The output intensity of the laser is monitored by a spectrometer. The monitored intensity decreases as the quantity of absorption species present in the sample increases.
Although laser spectroscopy has improved over the past few years, several disadvantages persist in conventional intra-cavity systems. By way of example, conventional systems tend to require expensive components and do not lend themselves to mass production. Conventional systems also tend to be too large to easily transport to field locations. Additionally, such systems typically require considerable adjustment to align the required optical components. Further, conventional systems typically may not provide a convenient method for tuning the laser cavity to examine the absorption characteristics of the absorber species over a plurality of frequencies.
Accordingly, an object of the present invention is to provide an inexpensive device for fiber laser intra-cavity spectroscopy.
Another object of the invention is to provide a compact device for fiber laser intra-cavity spectroscopy which is more convenient for field applications.
A further object of the invention is to provide a conveniently tunable fiber laser intra-cavity spectroscope.